Noticed the other day after the car was shut down with the trunk lid up to get the groceries that a hissing or white noise sound was coming from the sub woofer. The car had been turned off including the radio and sitting for ten minutes yet the sound was still there.

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I just answered my own question. I left the trunk open and came back after an hour and the sound was off. I suspect there must be some residual current flowing or retained to and by the sub which when depleted turns the speaker off.
As a side note, when I opened and closed one of the doors, the noise from the speaker reappeared.